Conventionally, a transmission device constituting a communication network, which is an address converting and transmitting device that converts and transmits network information stored in a packet has been proposed. Specifically, for example, when the address converting and transmitting device (for example, network address port translator (NAPT) and network address translator (NAT) (hereinafter, collectively NAPT), transmits a packet from a lower network configured at a level lower than the NAPT (for example, local area network (LAN)) to a higher network configured at a level higher than the NAPT (for example, wide area network (WAN)), a source address (private source address) specific to each terminal device, which is added to the packet upon sending of the packet by each terminal device connected via the lower network, is converted to an identical source address corresponding to the higher network (global source address) to transmit the packet.
A packet monitor acquires predetermined information from a packet transferred on a network (e.g., see Japanese Laid-open Patent Publication No. 2006-32997). Specifically, the packet monitor acquires a session ID, a source address, a source port number, a destination address, a destination port number, an identifier generated by adding a numerical value every time a packet is sent by a terminal device, and a serial number added at the time of dividing predetermined information into a plurality of packets and used at the time of reconstructing the information. The packet monitor acquires the session ID, source address, and serial number to calculate a packet loss rate by monitoring absence of a serial number for each session ID and source address. The packet loss rate calculated by the packet monitor is used to estimate a fault location of communications in the network by a network management system (NMS) or the like that manages the network.
However, in the above conventional technique, it is not possible to determine whether a source indicated by a source address is the NAPT or the terminal device.
That is, the packet monitor acquires the global source address and the private source address without identifying them. As a result, for example, when the NMS estimates a fault location of communications in the network, it is not possible to determine whether a source of a packet for which a packet loss has occurred is a terminal device connected via the NAPT. In other words, it has been difficult to cut and divide the fault location in the network between the higher network and the lower network by the mere packet loss rate measured by the packet monitor in the higher network.